[MINOR][ML] fix doc warnings
[MINOR][ML] fix doc warnings

Without an empty line, sphinx will treat doctest as docstring. holdenk

~~~
/Users/meng/src/spark/python/pyspark/ml/feature.py:docstring of pyspark.ml.feature.CountVectorizer:3: ERROR: Undefined substitution referenced: "label|raw |vectors | +-----+---------------+-------------------------+ |0 |[a, b, c] |(3,[0,1,2],[1.0,1.0,1.0])".
/Users/meng/src/spark/python/pyspark/ml/feature.py:docstring of pyspark.ml.feature.CountVectorizer:3: ERROR: Undefined substitution referenced: "1 |[a, b, b, c, a]|(3,[0,1,2],[2.0,2.0,1.0])".
~~~

Author: Xiangrui Meng <meng@databricks.com>

Closes #9188 from mengxr/py-count-vec-doc-fix.

JerryLead/spark,rikima/spark,ddna1021/spark,jrshust/spark,chuckchen/spark,esi-mineset/spark,eyalfa/spark,liyichao/spark,cloud-fan/spark,zhouyejoe/spark,wzhfy/spark,metamx/spark,eyalfa/spark,rednaxelafx/apache-spark,chuckchen/spark,actuaryzhang/spark,lxsmnv/spark,wzhfy/spark,panfengfeng/spark,alunarbeach/spark,ahnqirage/spark,zhouyejoe/spark,mike0sv/spark,zuotingbing/spark,jiangxb1987/spark,szhem/spark,sureshthalamati/spark,szhem/spark,nlalevee/spark,zero323/spark,HyukjinKwon/spark,sahilTakiar/spark,byakuinss/spark,zero323/spark,chuckchen/spark,HyukjinKwon/spark,techaddict/spark,loneknightpy/spark,zzcclp/spark,debugger87/spark,cin/spark,aosagie/spark,1haodian/spark,akopich/spark,Panos-Bletsos/spark-cost-model-optimizer,bravo-zhang/spark,kissmetrics/spark,xflin/spark,narahari92/spark,setjet/spark,apache/spark,ajaysaini725/spark,tengpeng/spark,cin/spark,jlopezmalla/spark,tengpeng/spark,ahnqirage/spark,JerryLead/spark,aokolnychyi/spark,lvdongr/spark,kiszk/spark,jkbradley/spark,ericvandenbergfb/spark,rezasafi/spark,hhbyyh/spark,1haodian/spark,map222/spark,aokolnychyi/spark,yanboliang/spark,gioenn/xSpark,kevinyu98/spark,nlalevee/spark,hvanhovell/spark,lvdongr/spark,ddna1021/spark,aokolnychyi/spark,spark-test/spark,jiangxb1987/spark,UndeadBaneGitHub/spark,rekhajoshm/spark,xflin/spark,WeichenXu123/spark,actuaryzhang/spark,spark-test/spark,stanzhai/spark,ddna1021/spark,wangmiao1981/spark,hhbyyh/spark,pronix/spark,nlalevee/spark,jlopezmalla/spark,dongjoon-hyun/spark,nilsgrabbert/spark,minixalpha/spark,jianran/spark,nlalevee/spark,eyalfa/spark,zzcclp/spark,liyichao/spark,aray/spark,kimoonkim/spark,gioenn/xSpark,1haodian/spark,vinodkc/spark,gioenn/xSpark,yanboliang/spark,witgo/spark,aokolnychyi/spark,WindCanDie/spark,Panos-Bletsos/spark-cost-model-optimizer,srowen/spark,shubhamchopra/spark,jlopezmalla/spark,janewangfb/spark,caneGuy/spark,big-pegasus/spark,nilsgrabbert/spark,witgo/spark,adrian-ionescu/apache-spark,kevinyu98/spark,xflin/spark,ibm-research-ireland/sparkoscope,joseph-torres/spark,ajaysaini725/spark,haowu80s/spark,liutang123/spark,apache/spark,caneGuy/spark,tejasapatil/spark,vinodkc/spark,Aegeaner/spark,joseph-torres/spark,sachintyagi22/spark,saltstar/spark,kiszk/spark,pronix/spark,liyichao/spark,mike0sv/spark,bOOm-X/spark,jiangxb1987/spark,minixalpha/spark,zuotingbing/spark,poffuomo/spark,cin/spark,spark-test/spark,ConeyLiu/spark,cloud-fan/spark,loneknightpy/spark,wangmiao1981/spark,yanboliang/spark,spark-test/spark,sahilTakiar/spark,shubhamchopra/spark,esi-mineset/spark,kissmetrics/spark,pgandhi999/spark,mike0sv/spark,shaneknapp/spark,rednaxelafx/apache-spark,ptkool/spark,skonto/spark,chuckchen/spark,eyalfa/spark,jiangxb1987/spark,minixalpha/spark,michalsenkyr/spark,janewangfb/spark,nilsgrabbert/spark,setjet/spark,huang1900/spark,facaiy/spark,bravo-zhang/spark,ptkool/spark,andrewor14/spark,ioana-delaney/spark,sachintyagi22/spark,haowu80s/spark,ron8hu/spark,zero323/spark,bOOm-X/spark,cloud-fan/spark,tejasapatil/spark,pgandhi999/spark,bOOm-X/spark,apache-spark-on-k8s/spark,mahak/spark,facaiy/spark,rekhajoshm/spark,dhruve/spark,liyichao/spark,rekhajoshm/spark,markhamstra/spark,pronix/spark,saltstar/spark,loneknightpy/spark,michalsenkyr/spark,holdenk/spark,bOOm-X/spark,skonto/spark,yanboliang/spark,ibm-research-ireland/sparkoscope,wangyum/spark,esi-mineset/spark,ajaysaini725/spark,shubhamchopra/spark,bravo-zhang/spark,saturday-shi/spark,ptkool/spark,tengpeng/spark,ueshin/apache-spark,mzl9039/spark,MLnick/spark,WeichenXu123/spark,mzl9039/spark,mzl9039/spark,stanzhai/spark,adrian-ionescu/apache-spark,rednaxelafx/apache-spark,taroplus/spark,LantaoJin/spark,SHASHANKB/spark,xflin/spark,byakuinss/spark,poffuomo/spark,sachintyagi22/spark,taroplus/spark,andrewor14/spark,ueshin/apache-spark,setjet/spark,akopich/spark,jrshust/spark,lxsmnv/spark,facaiy/spark,guoxiaolongzte/spark,Panos-Bletsos/spark-cost-model-optimizer,rezasafi/spark,big-pegasus/spark,UndeadBaneGitHub/spark,jkbradley/spark,gioenn/xSpark,MLnick/spark,WeichenXu123/spark,MLnick/spark,gengliangwang/spark,JoshRosen/spark,maropu/spark,someorz/spark,caneGuy/spark,jianran/spark,setjet/spark,rikima/spark,aosagie/spark,rekhajoshm/spark,alunarbeach/spark,darionyaphet/spark,SnappyDataInc/spark,nchammas/spark,icexelloss/spark,darionyaphet/spark,map222/spark,rezasafi/spark,dbtsai/spark,gengliangwang/spark,apache-spark-on-k8s/spark,tengpeng/spark,highfei2011/spark,ptkool/spark,tengpeng/spark,tejasapatil/spark,adrian-ionescu/apache-spark,witgo/spark,ericvandenbergfb/spark,icexelloss/spark,kissmetrics/spark,metamx/spark,shuangshuangwang/spark,WindCanDie/spark,pgandhi999/spark,poffuomo/spark,saltstar/spark,SHASHANKB/spark,xuanyuanking/spark,ibm-research-ireland/sparkoscope,SnappyDataInc/spark,tejasapatil/spark,ConeyLiu/spark,highfei2011/spark,aosagie/spark,ibm-research-ireland/sparkoscope,SnappyDataInc/spark,pronix/spark,maropu/spark,esi-mineset/spark,apache/spark,tejasapatil/spark,mahak/spark,panfengfeng/spark,rikima/spark,nchammas/spark,ericvandenbergfb/spark,caneGuy/s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